Stabilization of vinyl aromatic resins



STABHIIZATHON OF VINYL AROMATIC RESINS No Drawing. Application March 20,1940, Serial No. 325,050

Claims.

This invention concerns a method of stabilizing the resinous polymers ofvinyl aromatic compounds such as styrene, para-chloro-styrene,para-ethyl-styrene, ortho-chloro-styrene, orthomethyl-styrene,para-lsopropyl-styrene, etc., against discoloration .or oxidation duringexposure to light or oxidizing gases such as air. For convenience,resins oi the type just defined are hereinafter referred to genericallyas vinyl aromatic resins.

In a co-pending application Serial No. 294,934, filed September 14,1939, of ourselves, S. M. Stoesser and O. A. Braley, we have disclosedthe preparation of relatively stable vinyl aromatic resins bypolymerizing a vinyl aromatic compound, e. g., styrene, in the presenceof a minute proportion of a polyhydrlc phenol such as catechol ortertiary butyl catechol. In this co-pending application, less than 0.05per cent by weight of a polyhydric phenol is added to the vinyl aromaticcompound, after which the latter is polymerized by heating. Theresultant polymer is shown to be far more resistant to discolorationunder exposure to light and air than is a corresponding polymer preparedby polymerizing the vinyl aromatic compound alone. According to themethod of this co-pending application, it is necessary, however, thatthe polyhydric phenol be added in a proportion not greatly exceeding0.05 per cent of the weight of the vinyl aromatic compound, since theemployment of a larger pro portion of the phenol results in retardedpolymerization and also in the production of a lowmolecular weightpolymer of inferior strength. y

We have now found that similarly stabilized resins may be obtained byadding a polyhydric phenol to a vinyl aromatic resin after preparationof the latter and that when the addition is made in this manner,considerably larger proportions of the polyhydric phenol may be employedwithout impairing the properties of the polymer than is possible whenthe addition is made prior to polymerization as required in theabove-mentioned co-pending application. Also, a resin prepared bypolymerizing a vinyl aromatic compound in the presence of less than 0.05per cent of a polyhydrlc phenol as specified in said co-pendingapplication may be treated with additional polyhydrio phenol inaccordance with the present invention to further stabilize it againstdiscoloration by light or air.

In stabilizing a vinyl aromatic resin in accordance with the presentinvention a polyhydric phenol is incorporated as uniforml as possiblewith the resin. Stabilization is obtained regardless of the proportionor the manner in which the polyhydric phenol is added. However,

the phenolic compound is usually added in amount corresponding tobetween 0.03 and 3 per cent, preferably between 0.3 and 3 per cent, ofthe weight of the resin. The use of larger proportions, e. g., 5 percent or more, of the phenolic compound sometimes results indiscoloration due to oxidation of the phenolic compound itself uponprolonged exposure of the stabilized resin to air, particularly atelevated temperatures.

- The polyhydric phenol may be incorporated with the resin by mechanicalmixing at elevated temperatures on heated rolls or in other suitablemanner, or by dissolving the resin in a solvent such as benzene,toluene, xylene, or ethylbenzene, etc., adding the polyhydric phenol in,the desired proportion, and then evaporating the solvent. We have foundthat the polyhydric phenols may also conveniently be incorporated in avinyl aromatic resin by soaking the latter in a solution of thepolyhydric phenol in a liquid which is a solvent for the phenoliccompound but not for the resin. During such soaking operation theresin-absorbs the polyhydric phenol from the solution without itselfbecoming dissolved. Examples of liquid mediums which may be usedinbilized as just described may be devolatilized by' blowing heated airtherethrough without suffering discoloration or decomposition. Anunstabilized resin, e. g., polystyrene, when similarly devolatilized toremove unpolymerized styrene 4 and other volatile ingredients therefromis usually discolored markedly. However, polystyrene or other vinylaromatic resin, which has not previously been stabilized by treatmentwith a polyhydric phenol, may be devolatllized without sufferingappreciable discoloration by blowing the same with a heated sas, e. e.,air, nitrogen, carbon dioxide, etc., which contains a small proportionof a vaporized polyhydric phenol. During such operation, the resinabsorbs the polyhydric phenol from the gas stream and thusbecomesstabilized. The stabilized resins remain colorless during exposure atroom temperature to light and air for longer periods than docorresponding resins which have not been treated with a Dolyhydricphenol.

The following examples illustrate certain ways in which the principle ofthe invention has been applied, but are not to be construed as limitingits scope.

Example 1 A sample of pure granular polystyrene and another sample ofpolystyrene from the same source, which, however, had been treated with0.3 per cent by weight of para-tertiary-butyl-catechol, were each passedduring a period of minutes repeatedly through compounding rolls heatedto a temperature of approximately 150 C. At the close of this treatmentthe sample which contained no para-tertiary-butyl-eatechol was faintlyyellow, whereas the other sample remained clear and colorless andcontained the para-tertiary-butyl catechol uniformly incorporated withthe polystyrene. Each sample was then granulated. The granular materialswere each molded at a temperature of 160 C. to form test plates of thesame thickness. The test plates were placed equal distances from acarbon electrode arc lamp and were simultaneously exposed toultra-violet light from the lamp for 400 hours. At the close of thistreatment the plate of pure polystyrene was distinctly yellow, 1. e., itwas of deeper yellow color than before exposure to the light, whereasthe plate of polystyrene which contained para-tertiary-butyl catecholremained clear and colorless- Example 2 Several portions of granularpolystyrene were soaked in separate solutions ofpara-tertiarybutyl-catechol and ethanol for the respective periods givenin the following table. Each solution used in such soaking operationinitially contained 1 per cent by weight of para-tertiary-butylcatechol.After completing the soaking operations, the samples of granular resinswere filtered free of the liquor, dried, and molded at a temperature of160 C. into test plates of the same thickness. A similar test plate wasmolded from pure granular polystyrene. The transparency of each plate tovisible light was determined by exposing the plate to light from atungsten filament lamp operating at its normal voltage and measuring theintensity of the light transmitted through the plate. Each per centtransparency" value given in the table is the intensity of lighttransmitted through a plate expressed as per cent of the intensity ofthe light entering the plate. After such measurement of the transparencyof the freshly molded plates, the latter were aged by exposure toultra-violet light of sandard intensity for 400 hours. The aging, i. e.,discoloration due to exposure to light, which pure polystyrene undergoesduring 400 hours of exposure to light of the intensity used, correspondsapproximately to the aging which pure polystyrene of the same qualityundergoes upon outdoor exposure to sunlight for one year in the northernpart of the United States. After completion of the aging operation, eachplate was again tested to determine its transparency. The table givesthe time during which each sample of granular polystyrene was soaked inthe paratertiary-butyl-catechol solution. It also gives the initial percent transparency of each plate molded from the granular polystyrene andthe transparency of each plate after being aged as described above. Inthe table, the sample indicated as having 0 soaking time was the testplate molded from pure polystyrene which had not been treated withpara-tertiary-hutyl-catechol.

The purpose of this example is to illustrate the effectiveness ofpolyhydric phenols in inhibiting discoloration of plasticizedpolystyrene during heating of the latter in contact with air. A solutionof 1 part by weight of polystyrene and 2 parts of the plasticizingagent, ethylene glycol di-para-tertiary-butyl-phenoxy-acetate, and 7parts of benzene was prepared. A portion of this solution was used tocoat one face of several glass plates, after which benzene wasevaporated to leave deposited on the plates films of the plasticizedpolystyrene of between 0.002 and 0.003 inch thickness. Other portions ofthe above mentioned solution were treated with the respective polyhydricphenols named in the following table in the proportions also given. Eachproportion of a polyhydric phenol given in the table is the per cent byweight of such compound, based on the weight of polystyrene present. Thesolutions of polystyrene and polyhydric phenols were cast as films onglass plates as above described. Each plate was heated in contact withair at a temperature of C. for 3 hours. The plates were then cooled andtested to determine their transparency toward ultra-violet light. Thistest was carried out by exposing each plate to ultra-violet light ofstandard intensity and measuring the intensity of the light transmittedthrough the plate. The "per cent U. V. transparency" of each test plategiven in the table is the intensity of the ultra-violet lighttransmitted through the plate expressed as per cent of the intensity ofthe light entering the plate. The glass plates upon which the films werecast were equally transparent to ultra-violet light; hence thetransparency values given in the table indicate the relativetransparencies toward ultra-violet light of the films cast upon theplates. It may be mentioned that under these test conditions a reductionin transparency of a film appears to be due largely to discoloration ofthe film through oxidation by air during the heating operation.Accordlngly, the films having the largest transparency values are thosewhich are most resistant to such oxidation and resultant discoloration.The transparency value for film of pure polystyrene alone which is givenin the table is an asoacoc average of the values obtained in four testsof such film.

TABLE 2 Polyhydric phenol Percent Run U. V.

No. trans- Kind Cone parency 1 None 26 2 Catcchol 0. l 32 3 .....do 0.538 4 d0. 3.0 44 5 RcsorcinoL 0.1 3'. (i rin 0.5 34 7 "do .l 3.0 30 8Hydroquiuonc. 0.1 31 9 ..do.... 0,5 43 10 .JJO l 3.0 27 ll B.B-Bis.-(4-hy lroxy-phenyl) -isopro- 0.1 32

pane.

I2 (ln. 0.5 44 Iii ..do l 3.0 5i ]i 0.0-l)i-hydroxy-henznnhr-nono 3. 045 p-Tertiary-hutyl-catechnl 0.02 38 I; .1n 0.1 45 17 .110 0 5 40 18 do3. 0 75 Other polyhydric phenols which may be employed as stabilizingagents for vinyl aromatic resins in accordance with the invention arepyrogallol, 3-methyl catechol, 3.5-diethyl catechol, tertiary-amylcatechol, etc. All polyhydric phenols are useful as stabilizing agentsfor vinyl aromatic resins in accordance with the present method, butcatechol and the nuclear \alkylated catechols are particularlyefiective. For convenience, catechol and the nuclear alkylatedcatecholsare referred to generically, in certain of the claims, as catecholcompounds.

Other modes of applying the principle of the invention may be employedinstead of those explained, change being made as regards the methodherein disclosed, provided the step or steps stated by any of thefollowing claims or the equivalent of such stated step or steps beemployed.

We therefore particularly point out and distinctly claim as ourinvention:

1. The method of stabilizing a vinyl aromatic resin againstdeterioration under exposure to light and air which comprisesincorporating therewith a minor proportion of a polyhydric phenol.

2. The method of stabilizing a vinyl aromatic resin against oxidationwhich comprises incorporating therewith between 0.03 and 3 per cent byweight of a polyhydric phenol.

3. The method of'stabilizing a vinyl aromatic resin which comprisesincorporating therewith between 0.3 and 3 per cent of a catecholcompound.

4. The method which comprises incorporating a minor proportion of apolyhydric phenol with a vinyl aromatic resin which contains anunpolymerized vinyl aromatic compound as an impurity and thereaftercontacting the resin with a current of aheated gas to vaporize andremove the unpolymerized vinyl aromatic compound therefrom.

5. The method which comprises contacting a vinyl aromatic resin whichcontains a volatile impurity with a current of a heated gas containing asmall proportion of a vaporized polyhydric phenol, whereby the volatileimpurity is vapor,- ized and removed from the resin and the latterabsorbs the polyhydric phenol from the gas stream and thus becomesstabilized.

6. The method which comprises contacting polystyrene which containsstyrene with a current of heated air containing a vaporized catecholcompound, whereby styrene is vaporized and removed from the polystyreneand the latter absorbs the polyhydric phenol from the gas stream andthus becomes stabilized.

7. The method of stabilizing a vinyl aromatic resin which comprisescontacting the resin with a solution of a polyhydric phenol in a liquidv which is a non-solvent for the resin, whereby the latter absorbs thepolyhydric phenol from the solution.

8. The method of stabilizing polystyrene which comprises incorporatingtherewith between 0.3 and 3'per cent by weight of a catechol compound.

9. The method of stabilizing polystyrene which comprises contactingpolystyrene with an alcholic solution of a catechol compound, wherebythe polystyrene absorbs the polyhydric-phenol from the solution.

10. The method of stabilizing polystyrene which comprises incorporatingtherewith between 0.3 and 3 per cent of para-tertiary-butyl- LORNE A.MATHESON. RAYMOND F. BOYER. JAMES L. AMOS.

unpolymerized

